System and method for reducing draw resonance in plants for the production of plastic film

ABSTRACT

A system and method for reducing draw resonance of plastic material in the molten state, so-called melt, leaving an extrusion group of a plant for the production of plastic film, includes at least one thermostatically-controlled cylinder, having an embracement angle, of the melt on the cylinder, adjustable on the basis of the process rate, i.e. the linear movement speed of the plastic film, and/or on the basis of the temperature measured in the proximity of or in correspondence with the clamping area of the melt in a thermoforming, calibration and cooling group included in the plant and positioned downstream of the system.

FIELD OF THE INVENTION

The present invention relates to a system and a method for reducing thedraw resonance in extrusion plants having the cast technology for theproduction of plastic film.

BACKGROUND OF THE INVENTION

It is known that in plants with cast technology, downstream of one ormore single-screw or twin-screw extruders, there is at least one coolinggroup, typically but not exclusively composed of one or more cylinders,cooled or thermoregulated by means of any carrier fluid (water, oil,air, etc.), either inside or outside the cylinder itself. The extrudedplastic film comes into contact with said cooling group after passing alonger or shorter section “in air”, free, i.e. without any contact withany other element of the production line.

This section “in air” is covered with the molten mass of plasticmaterial still in melt form, i.e. without any specific form and with thepossibility of being geometrically deformed, in each of the threedirections, by the application of forces even of an extremely slightentity.

This causes the specific form of the melt to also be influenced byexternal elements such as air currents, surface irregularities of themelt itself, etc., in addition to considerably amplifying theconsequences of possible defects in thickness.

The very nature of the melt makes it practically impossible to apply aconstant tension, i.e. any tension that is applied cannot be transmittedhomogeneously.

The consequence of this behaviour of the melt is so-called “drawresonance” i.e. a vibration/resonance caused by a continuous variationin the geometry of the melt induced by the non-homogeneous cooling ofthe melt in said section “in air”, in turn caused by the tensiongenerated by the cooling group situated immediately downstream of theextrusion elements.

Draw resonance is a phenomenon which is particularly harmful for thequality of the plastic film produced in the plant.

This phenomenon is basically present regardless of the type of polymerextruded, whether it be of a polyethylene or polypropylene matrix, orbased on polyamide or polystyrene, or containing organic or inorganicfillers such as calcium carbonate or glass fiber.

This occurs because this phenomenon is not related to thecrystallization process (in turn greatly characterized by the type ofpolymer used), but is due to the geometry of the system, i.e. the typeof technology adopted for the production of plastic film.

Draw resonance is also evident in extrusion plants that use the blowntechnology, wherein the whole cooling process of the film is effected“in air”, therefore without the use of any cooling means by contact, thephenomenon however is enormously amplified in plants with the casttechnology as:

in the case of lines based on the blown technology, the rate of theprocess (i.e. the linear movement speed of the plastic film) is, atbest, close to or slightly higher than 200 m/min, whereas in lines basedon the cast technology the rate of the production line is well over 300m/min, and in some cases even 400 m/min;

in the case of plants based on the blown technology, the extruded meltin any case has a continuous form (commonly called “balloon”), whichallows a perfect symmetry of the internal tensions, also when the sameare extremely weak, as the polymer is still in the molten state, whereasin the case of production lines based on the cast technology, on thecontrary, in addition to the problems indicated above, there is also asignificant asymmetry of the extruded film, which, in the extrusiondirection, has two edges inevitably subjected to forces different fromthe rest of the film itself, and tending towards the centre of the same(i.e. the so-called “neck-in” effect).

Typical examples of production lines based on the cast technology inwhich the draw resonance phenomenon has a fundamental role in limitingthe production capacity, are the so-called “coating” lines, in which themelt is poured directly onto the surface of one or two external elementssuch as paper, fabrics, aluminium or other polymers, and simultaneouslypasses through a so-called “clamping”, i.e. two cylindersthermoregulated with any carrier fluid in contact with each other, so asto ensure the joining of said melt with the above-mentioned externalelements.

The merging of said melt with the external elements must take placeunder well-defined thermal conditions, i.e. the temperature of the meltat the moment of merging must be high enough to ensure the correctchemical reaction between the molecules of the various elementsinvolved, but at the same time must be sufficiently low as to allow anadequate regulation and management of the layer of melt that is beingdeposited on said elements.

Another typical and very common case in which the draw resonancephenomenon has an even more marked importance which is limiting forproduction, is that of so-called “melt embossed” lines.

In practice, in these production lines, the melt is poured directly intothe so-called “clamping”, as in the case previously described, in whichthe first cylinder consists of a metallic element (typically steel) witha particular surface design, so-called “matte coating” or “embossing” or“pattern” produced with a wide variety of surface machining methods(spark erosion, machining by chip removal, blasting, etc.), whereas thesecond cylinder has a surface with a rubber coating, of the most variedquality (silicone rubber, polyurethane, etc.), and is pressed againstthe first cylinder by means of any contact system (pneumatic, hydraulic,electronic, or any other).

The “melt embossed” process therefore consists in permanently deforminga surface of the final film, but possibly both surfaces, by means of amechanical deformation action ensured by the thrust of the rubberizedsurface, and therefore deformable, of the second cylinder against therigid and substantially non-deformable surface of the first cylinder,simultaneously also effecting the final cooling of the film so as tomake the deformation effective, which, in the case of excessivetemperature of the melt, would otherwise be “cancelled” by the elasticreturn.

The final film thus acquires the same form as the surface of theso-called embossed cylinder, and can therefore have the most variedforms (pyramid, cone, semispherical, etc.).

This technology is basically (but not exclusively) used in the field ofhygiene applications (e.g. diapers for children and adults), as thissurface gives the plastic film a tactile and visual characterizationthat makes it resemble a fabric, with beneficial effects on thesensation of whoever is using the end-product.

As in the case of so-called “coating” production lines, the definitionand management of the temperature of the melt at the moment in which theembossing is effected, is extremely important also for “melt embossed”lines, due to the necessity of applying a permanent deformation, whichcan only be such if it is carried out at a temperature close to theVicat value. As the cooling of the plastic film, from the extrusiontemperature (indicatively but not exclusively around 200-250° C.) up toa value close to the Vicat temperature (which, depending on the type ofpolymer used, can indicatively range from less than 100° C. to over 150°C.), is only ensured by the section “in air” along which the melt passesfrom the outlet of the extrusion die to the “clamping” between the twocylinders, it is obvious that with an increase in the production rate,this distance must currently also be increased, so as to give the melttime for lowering its temperature to the desired value.

This operation, which is extremely simple from a mechanical point ofview, as it basically amounts to adequately dimensioning the geometry ofthe area of the extrusion die and the casting and “clamping” group,involves, however, a series of drawbacks and problems which effectivelylimit the maximum production rate that can be reached with this type ofplant. This operation in fact causes:

an exponential increase in the so-called “neck-in effect” (i.e. thenarrowing of the width of the falling melt due to the Van-der-Waalsintermolecular forces which tend to bring the molecules together untilthey acquire a form having a lower potential) which entails thenecessity of considerably increasing the width of the extrusion die withan increase in the production rate in order to reach the same finalwidth of the film;

a greater sensitivity of the falling melt to any type of external factor(air currents, heat sources, etc.), due to the larger surface exposed;

the onset, above all, of an evident draw resonance.

Various solutions are known in the state of the art for cooling themelt, which, however, do not guarantee an adequate constancy and controlof the temperature of the melt, such as, for example, an air blow thatmeets the melt in the free section in which there is no contact with theelements of the production line.

Although this solution offers advantages, it has the drawback of notallowing accurate and repeatable adjustments, leaving the success of theproduction of plastic film up to the operator's experience.

Document U.S. Pat. No. 4,608,221 describes a process for reducing the“draw resonance” phenomenon in which the film or strip, before reachingthe two cooled “clamping” cylinders, slides on the arched surface of acylinder of a tensioning device, which can be moved so that the tensionon the film remains approximately constant. This is obtained with asubstantially friction-free surface, thanks to the choice of material(for example Teflon) or preferably with a surface on which air, steam oranother fluid is directed, which prevents contact between the strip orfilm and the surface of the cylinder.

The tensioning device is positioned so that the surface, free offriction, exerts a tensioning effect in the width direction against themoving strip, in a point between the extrusion and the cooling area.

The cylinder extends in the width direction of the strip and has aportion of its surface, adjacent to the strip, which is permeable, sothat the air (or another fluid), entering the cylinder, can exit and beused for keeping the molten strip separated for at least about 0.1 inch,from the solid surface of the cylinder. The fluid leaves the surface ofthe cylinder at such a rate as to avoid causing distortions but only acushioning effect. In this way, the strip and the cylinder can be movedsimultaneously, in a direction towards or away from a certain position,according to an increase or decrease in the tension of the strip, sothat fluctuations in the tension can be amortized.

The tensioning device is provided with a counterweight, a spring or thelike, which allow an appropriate tension of the cylinder with the moltenstrip to be maintained in order to reduce the occurrence of the “drawresonance” phenomenon.

The method described above therefore provides for reducing the drawresonance by simply amortizing the movements of the strip which ismoving from the extruder towards the thermoforming and cooling area.

SUMMARY OF THE INVENTION

The general objective of the present invention is to provide a systemand method for an extrusion plant of film made of plastic material, thatallows the regulation and accurate management of the temperature of themelt in correspondence with the “clamping” cylinders, in order to avoidthe drawbacks described above and allow not only an increase inperformance of the production lines but also a greater constancy and,ultimately, a higher quality of the film produced.

In particular, the objective of the present invention is to provide asystem and a method which reduces the draw resonance phenomenon inproduction plants of plastic film, in particular cast technology plants.

An object of the present invention therefore relates to a system and amethod for reducing the draw resonance of plastic material in the moltenstate, so-called melt, leaving an extrusion group of a plant for theproduction of plastic film wherein said system comprises at least onethermostatically-controlled cylinder, having an angle of embracement ofthe melt on said cylinder, adjustable on the basis of the process rate,i.e. the linear movement speed of the plastic film and/or on the basisof the temperature measured in the proximity of or in correspondencewith the clamping area of said melt in a thermoforming, calibration andcooling group included in said plant and positioned downstream of saidapparatus.

An object of the present invention also relates to a plant, inparticular a cast plant for the formation of film made of plasticmaterial comprising said system for reducing the draw resonance.

BRIEF DESCRIPTION OF THE DRAWINGS

The structural and functional characteristics of the present inventionand its advantages with respect to the known art will appear even moreevident from the following description, referring to the enclosedschematic drawings, which show an embodiment example of the sameinvention:

In the drawings:

FIG. 1 is a transversal section of an extrusion plant of plastic filmprovided with a system according to the present invention;

FIG. 2 is a longitudinal section of a system according to the presentinvention;

FIG. 3 is a transversal section of an extrusion plant of plastic filmprovided with a system according to the present invention illustratingthe two limit positions for the embracement angle of thethermostatically-controlled cylinder.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

With reference to the figures, FIG. 1 shows an extrusion plant for theproduction of film made of plastic material comprising at least oneextrusion group 2 and, downstream of said extrusion group 2, at leastone thermoforming, calibration and cooling group 3, for the formation ofa mono-or multi-layer film.

According to the present invention, said plant is provided with a system1 for the temperature regulation of the melt positioned upstream of saidat least one thermoforming, calibration and cooling group 3.

Said apparatus can be provided in plants with a flat-die extrusiontechnology (cast film).

The system 1 object of the present invention, which allows a reductionin the draw resonance of the plastic material in the molten state,so-called melt 12, leaving an extrusion group 2 of a plant for theproduction of plastic film comprises at least onethermostatically-controlled cylinder 10, having an embracement angle A,of the melt 12, falling from the extrusion group 2 onto said cylinder10, adjustable on the basis of the process rate, i.e. the linearmovement speed of the plastic film and/or on the basis of thetemperature measured in the proximity of or in correspondence with theclamping area 13 of said melt 12 in a thermoforming, calibration andcooling group 3 included in said plant and positioned downstream of saidsystem 1.

Said cylinder 10 is preferably made of steel.

According to an embodiment, the temperature regulation of the cylinder10 is effected through a heat-transfer fluid in the liquid state, suchas water, oil or the like, conveyed by a rotating joint 11, positionedcoaxially inside said cylinder 10.

Alternatively, the regulation of the surface temperature of the cylinder10 can be effected by means of a heat-transfer fluid in the gaseousstate.

The blowing of air or vapour inside the cylinder 10 can be provided,using any suitable means, such as a compressor or fan, creating a flowwith an adjustable temperature and flow-rate, as said cylinder 10 isprovided on at least part of the mantle with holes for the outflow ofsaid air or vapour.

The blowing of air or vapour can be effected from one or both of thebases of said cylinder 10, to be discharged from the holes directlyagainst the surface of the melt 12 falling from the extrusion group 2,thus causing a controlled cooling thereof and consequently eliminatingthe “draw resonance” phenomenon.

The flow of fluid, such as oil, water or the like, allows the surfacetemperature regulation of the cylinder 10 in an operating range that canvary from 10° C. to 200° C.

The system 1 comprises regulation means of the horizontal movementand/or regulation means of the angular movement of said cylinder 10.

Said embracement angle A can be varied:

by means of a horizontal movement within guides 14 secured by a pair ofadjustment screws 15,

by means of an angular movement by rotation of the supporting levers 16of the cylinder 10 around a fulcrum 17, by means of a second pair ofadjustment screws 18.

The combination of the horizontal and angular movements allows theembracement angle A of the melt 12 on the cylinder 10 to be consistentlyvaried.

As illustrated in FIG. 3, the embracement angle A can be preferablyvaried from 0° (no contact) up to about 180°.

The variation in the embracement angle A and the management of thesurface temperature of said cylinder 10 therefore ensures, with greataccuracy, the management of the temperature of the melt 12 at the momentof clamping 13, as it allows heat to be supplied or removed.

Said cylinder can be either rotating or non-rotating.

The motorization of the cylinder 10 is ensured by an externalmotorization 19 connected to the same cylinder 10, preferably but notexclusively, by means of a universal joint 20, which guarantees themanagement of the rate of the cylinder 10 independently of the otherrotating elements, forming the extrusion and production line of plasticfilm.

This motorization can in any case also be adopted for other rotatingelements of the extrusion and production line of plastic film, such asone or both cylinders forming the clamping 13, by means of transmissionbelts or any other known movement transmission system.

As an alternative to a rotating cylinder 10 and therefore motorized, anon-rotating cylinder 10, and therefore fixed, can be provided, butwhose position can be equally adjusted analogously to what is describedabove.

According to an embodiment, a non-rotating cylinder can be provided,thermoregulated by means of an air flow: this embodiment avoids the useof both the motor 19 and the consequent universal joint 20, and also thehydraulic joint 11 for the passage of the carrier fluid, reducing thecylinder 10 to a kind of “tube” through which air passes, blown by afan, a compressor or any other system suitable for creating awell-defined flow of air, whose flow-rate and temperature can beregulated.

An object of the present invention also relates to a plant for theproduction of film made of plastic material comprising:

at least one extrusion group 2;

at least one thermoforming, calibration and cooling group 3;

downstream of said at least one extrusion group 2, at least a system 1for reducing the draw resonance of the plastic material in the moltenstate, so-called melt 12, leaving an extrusion group 2 produced asdescribed above.

The plant is preferably a plant for the formation of flat-die film, aso-called cast film plant.

According to the present invention, a detection sensor of thetemperature of the melt 12 is provided in correspondence with or in theproximity of the clamping area 13.

The temperature of the melt 12 is measured with great accuracy by meansof a temperature sensor which exploits the known infrared technology, orusing any other detection system; said temperature measurement istherefore used as reference for:

managing the embracement angle A of the cylinder 10 with the melt 12;

regulating the temperature of the cylinder 10 by adjustment of thetemperature and/or flow-rate of the cooling fluid.

A further object of the present invention relates to a method forreducing the draw resonance of plastic material in the molten state,so-called melt 12, leaving an extrusion group 2 of a plant for theproduction of plastic film wherein, between the extrusion step of themelt 12 and the clamping step 13 effected by at least one thermoforming,calibration and cooling group 3 of the plant, there is a temperatureregulation phase of the melt 12 effected through the passage of saidmelt 12 over at least one thermostatically-controlled cylinder 10,having an embracement angle A, of the melt 12 on said cylinder 10,adjustable on the basis of the process rate, i.e. the linear movementspeed of the plastic film and/or on the basis of the temperaturemeasured in the proximity of or in correspondence with the clamping area13 of said melt 12.

The forms of the structure for implementing the system and method of thepresent invention, as also the materials and assembly modes, cannaturally differ from those shown for purely illustrative andnon-limiting purposes in the drawings.

The protection scope of the present invention is defined by the enclosedclaims.

The invention claimed is:
 1. A system for reducing draw resonance of amelt of plastic material in molten state, leaving an extrusion group ofa plant for production of plastic films, comprising: at least onethermostatically-controlled cylinder, having an embracement angle of themelt on the thermostatically-controlled cylinder, adjustable based onone or more of: process rate, the process rate comprising a linearmovement speed of a plastic film, or temperature measured in proximityof or in correspondence with a clamping area of said melt in athermoforming, calibration and cooling group included in said plant andpositioned downstream of said system.
 2. The system according to claim1, wherein temperature regulation of said cylinder is performed througha heat-transfer fluid in liquid state conveyed by a rotating jointpositioned coaxially inside said thermostatically-controlled cylinder.3. The system according to claim 1, wherein temperature regulation ofsaid thermostatically-controlled cylinder is performed through aheat-transfer fluid in gaseous state that flows from one or both basesof said cylinder and exits from holes provided on at least part of amantle of thermostatically-controlled said cylinder.
 4. The systemaccording to claim 1, further comprising one or more of a regulationsystem of a horizontal movement or a regulation system of an angularmovement of said thermostatically-controlled cylinder.
 5. The systemaccording to claim 1, wherein the embracement angle ranges from 0° to180°.
 6. The system according to claim 1, wherein the temperature ofsaid cylinder ranges from 10 to 200° C.
 7. The system according to claim1, wherein said thermostatically-controlled cylinder is rotating ornon-rotating.
 8. A plant for production of a film made of a plasticmaterial comprising: at least one extrusion group; at least onethermoforming, calibration and cooling group; and downstream of said atleast one extrusion group, at least one system reducing draw resonanceof a melt of the plastic material in molten state, leaving the extrusiongroup comprising: at least one thermostatically-controlled cylinder,having an embracement angle of the melt on thethermostatically-controlled cylinder, adjustable based on one or moreof: process rate, the process rate comprising a linear movement speed ofthe film, or temperature measured in proximity of or in correspondencewith a clamping area of said melt in a thermoforming, calibration andcooling group included in said plant and positioned downstream of saidsystem.
 9. The plant according to claim 8, wherein the plant is adaptedto form flat-die film, causing the plant to be a cast film plant. 10.The plant according to claim 8, wherein, in correspondence with or inproximity of the clamping area, there is a detection sensor of thetemperature of the melt.
 11. A method of reducing draw resonance of amelt of a plastic material in molten state, leaving an extrusion groupof a plant for production of a plastic film, comprising: between anextrusion step of the melt and a clamping step performed by at least onethermoforming, calibration and cooling group of the plant, causing atemperature regulation step of the melt performed through passage ofsaid melt over at least one thermostatically-controlled cylinder, havingan embracement angle, of the melt on said cylinder, adjustable based on:process rate, the process rate comprising one or more of a linearmovement speed of the plastic film, or temperature measured in proximityof or in correspondence with the clamping area of said melt.